Materials which do not deteriorate mechanically and continue to function in extremely high temperatures, say from 300.degree. C. to as high as 2500.degree. C., or even higher, are desirable and necessary in a wide range of modern applications.
Examples of such applications include those in the aerospace industry where numerous structures have such requirements, including rocket and space shuttle nose cones, re-entry heat shields, rocket and jet engine nozzles, and leading edges of aircraft and spacecraft. Brake systems, particularly for high speed vehicles, and turbine components also encounter such conditions. The uses for such materials are expected to increase in the future. Indeed, each advance in material science increases the possible applications for materials which are stable at increasingly high temperatures.
Ceramics have replaced metals in some such applications, the most popularly known of which is the ceramic tiles used as a re-entry heat shield for the space shuttle. However, it is well-known that these tiles suffer severe injury during each re-entry and many must be replaced after each expedition.
One class of materials useful in such applications has been composites of carbon fibers reinforcing a carbon matrix. These may conveniently be referred to as CFCM composites. Co-inventor J. Economy, with H. Jung and T. Gogeva, authored a paper describing processes for preparing CFCM composites in Carbon, Vol. 30, No. 1, pp 81-85 (1991).
CFCM composites are considerably stronger and lighter than graphite. They actually increase in strength with increasing temperature and resist thermal shock caused by rapid ambient temperature changes. However, fabrication is a slow expensive process. The carbon matrix is usually introduced among the carbon fibers by liquid impregnation and charring of organic materials. In some applications, chemical vapor deposition is used as a final step in processing. The steps in the processes are repetitive and can take months to complete.
Coatings are often required to protect the carbon containing composite from oxidation at high temperatures. See, for example, the article on such coatings by H. Jung and J. Economy in Polymers for Advanced Technology, Vol. 2, pp 265-269 (1991). Coatings may be less effective than desired due to the development of pinholes or microcracks in the coatings or diffusion of oxygen through the coating.
Another composite was described by R. Lin, J. Economy and H. Batha in Ceramic Bulletin, Vol. 55, No. 9, pp 781-784 (1976). This composite consisted of a boron nitride matrix reinforced with boron nitride fiber. Boron nitride fibers are described in U.S. Pat. No. 3,620,780 to J. Economy and R. Andersen. Boron nitride would appear to have advantages for structures in a high temperature environment due to its high thermal resistance and other properties. In the cited study, boron nitride fibers were mixed with partially nitrided boron nitride fibers and the mixture was hot pressed from room temperature to 1400.degree. C. and then to 2000.degree. C. The resulting composites were extremely brittle which may explain why such composites have not been the subject of any subsequent research known to applicants.
In a paper published in Chemistry of Materials 1990, 2, pp 96-97, P. J. Fazen, et al. reported the production of a boron nitride composition from borazine. Both the method and the resulting composition are different from those of the present invention. The boron nitride produced by the process typically had an interlayer spacing of 3.55 Angstroms to 3.59 Angstroms. However, for boron nitride to have acceptable resistance to deterioration caused by moisture the interlayer spacing must be much lower, in the range of 3.35 Angstroms to 3.40 Angstroms.
The present invention overcomes many of the problems of the prior art which are discussed above. By providing an oligomeric precursor of boron nitride with appropriate viscosity, a boron nitride precursor is produced which can wet and impregnate a variety of fiber layups used in structural composites. Preparation time for such composites is shortened from months to days with concomitant cost savings. Composite characteristics may be easily adjusted for particular applications.